ATM leased line services, such as an ATM Mega-link service, that are currently available and served by the ATM (Asynchronous Transfer Mode), provide a user with a pipe of a virtual path (VP), and realize a conventional leased line on ATM.
Conversely, an ATM Share-link service which is a lower-priced ATM leased line service gives a user a virtual channel (VC) by guaranteeing only the minimum bandwidth and sharing the bandwidth beyond it with other users to lower cost for the service.
That is, by contracting the ATM Mega-link service in increments of a virtual path, a user can set up an arbitrary number of virtual channels. Conversely, the ATM Share-link service provides is available in increments of a virtual channel, and the user thereof cannot manage an arbitrary number of virtual channels.
For this reason, if a bandwidth sharing with a minimum bandwidth similar to the ATM sharing service can be performed in each virtual path, the user can enjoy an economical service by setting up virtual channel connections freely in the virtual path, using MPOA (MultiProtocol Over ATM) and the like, and by sharing the bandwidth with other users.
In this case, in order to perform a frame-by-frame processing, such as EPD (Early packet Discard) and the like provided with a minimum bandwidth guarantee, a network side has to recognize boundaries of frames. However, so long as a contract is made on a virtual-path-to-virtual-path basis, how virtual channels are multiplexed therein is unknown. Since cell multiplexing is generally carried out as shown in FIG. 1(B), it is difficult to recognize the boundaries of the frame for every virtual channel, unless a terminal side outputs frame by frame as shown in FIG. 1(A). Here, in reference with FIG. 1(A) and (B), a square expresses a cell, a black square expresses a virtual channel 1 of a frame 1, and a virtual channel 1 of a frame 2, and a white square expresses a virtual channel 2 of a frame 1, and a virtual channel 2 of a frame 2.
For this reason, in the ATM Share-link service, a frame information management table is provided to every virtual channel, and each frame is processed while checking whether a cell represents a head, middle or end of the frame in a buffer that performs bandwidth sharing. Here, an example of the frame processing is an EPD shown in FIG. 2 and the like. If the number of cells stored in a queue is not over a threshold when the head cell of the frame arrives as shown in FIG. 2(A), the EPD inputs all cells of the arriving frame into the queue, even if the number of the cells exceeds the threshold on the way. If the number of cells stored in the queue is over the threshold when the head cell of the frame arrives as shown in FIG. 2(B), all cells of the arriving frame, from the head cell to the end cell, will be discarded.
Here, the problem is that a VCI (Virtual Channel Identifier) is unknown because a choice of VCI is at the discretion of users in the contract of a virtual path, causing an inability to perform the frame information management for every virtual channel as described above, which is possible, on the other hand, in the ATM Share-link service because it is a contract for a virtual channel.
As regards the frame processing of EPD and the like, if a virtual channel is prepared beforehand as in a PVC (Permanent Virtual Connection), only a preparation of a management table will be necessary. However, this management table may become large depending upon processes, making it difficult to set up a large number of virtual channels.
In order to realize the EPD, it is necessary to control writing using information about frame position (whether a cell is a head or a middle of a frame) and information about cell memory queue size. FIG. 3 is a block diagram of an example of a conventional EPD processing circuit. In this drawing, a writing control unit 10 discards a head cell and following cells in the same frame, if a queue size of a cell memory 12 is over a predetermined threshold value when the head cell of the frame arrives. If the head cell of the frame arrives when the queue size of the cell memory 12 is not over the predetermined threshold, the head cell and following cells in the same frame will be inputted.
Whether the head cell of the frame was inputted or discarded is managed by a VC management memory 14. The writing control unit 10 decides whether to input or discard at the frame head, the decision thereof is notified to the VC management memory 14, and at other than the head of the frame, the decision information is conversely obtained from the VC management memory 14, and an input to the cell memory 12 is controlled.
In this manner, the VC management memory 14 manages the frame position information and the head cell processing information for each virtual channel, wherein a VPI/VCI (virtual channel identifier, VPI: Virtual Path Identifier) value to be used is set up either by an operator or by a signaling processing unit (set up by a control system in either case), and the frame information of cells that belong to the virtual channel corresponding to the value is managed.
That is, the scalability of the number of virtual channels for frame processing may be restricted by a capacity of the VC management memory. There has been a problem that a table that can process a great number of virtual channels even if there are few virtual channels actually communicating has to be prepared, if virtual channels have to be set up beforehand as conventionally.